feat(cli): truthful rendering for background shell tasks #1096

Problem

A Bash call with run_in_background: true returns immediately with a maka://runtime/background-tasks/<id> ref (shell-run-contract.ts). The TUI settles the Bash card as done while the process still runs, and the model's subsequent polling of the ref renders as internal Read cards in the transcript. The card lies about the process state, and the polling leaks into the user-visible transcript.

Desired outcome (MVP)

• The parent Bash card stays visibly running (elapsed time, latest output tail, interrupt hint) until the process settles, then shows a final done / failed / aborted state.
• Ref polling folds into the parent card instead of rendering Read cards for background-tasks refs.
• This lands the live, event-driven tool-card mechanism once; the subagent TUI surface (filed separately) will reuse it.

Explicitly out of scope

User-facing interaction: statusline indicator, focusing a task to inspect progress, /background. File a follow-up if real pain emerges.

(Split out of the retired #549 tracker.)